How to get 2.86 seconds in this kinematics problem

[Jacob T Anderson]

This is a worked problem, so they're just basically giving examples on how you would solve this type of problem. My issue is... how did they get 2.86s? Please someone help this has been driving me crazy.

 

[Bandersnatch]

Hi, Jacob. Can you rearrange the equation so that it has just t on one side?

 

[Jacob T Anderson]

Hi, Jacob. Can you rearrange the equation so that it has just t on one side?

It mentions about rearranging the equation but I don't think it shows you. Do you know what it would look like if it was rearranged? If so please show me.

 

[Bandersnatch]

The forum rules prohibit us from giving you the answer. You have to show your work.

If you're working from this textbook, you should already know how to rearrange algebraic equations.
The equation is
$$40=0+\frac{1}{2}9.8*t^2$$
How would you move all the numerical values to the left?

 

[Ray Vickson]

It mentions about rearranging the equation but I don't think it shows you. Do you know what it would look like if it was rearranged? If so please show me.

No, we cannot do that; it is contrary to the PF rules. You MUST attempt to solve the problem first, and show your work.

 

[Jacob T Anderson]

The forum rules prohibit us from giving you the answer. You have to show your work.

If you're working from this textbook, you should already know how to rearrange algebraic equations.
The equation is
$$40=0+\frac{1}{2}9.8*t^2$$
How would you move all the numerical values to the left?

Wouldn't you just 0.5 * 9.81 + 0 = 4.905t². Then that would give you 40 = 4.905t²

 

[Jacob T Anderson]

Wouldn't you just 0.5 * 9.81 + 0 = 4.905t². Then that would give you 40 = 4.905t²

But that still doesn't make sense because when you divide afterwards it still doesn't give you the answer.

 

[Bandersnatch]

Wouldn't you just 0.5 * 9.81 + 0 = 4.905t². Then that would give you 40 = 4.905t²

This is not algebraically correct.

The rule is this:
Let's say you have an equation with four variables or constants: a,b,c,d or the form:
$$a*b=c*d$$
if you want to find d, you must eliminate everything but d from one side of the equation. This can be done by dividing both sides by the same value. Since we want to get rid of c on the right hand side, we can divide both sides by c:
$$\frac{a*b}{c}=\frac{c*d}{c}$$
$\frac{c}{c} = 1$, so we end up with:
$$\frac{a*b}{c}=d$$

Can you apply the same transformation to the equation given in the problem?

 

[CWatters]

40=4.905t² is correct and if you keep going it does give you the right answer.

Show your working so we can see where you are going wrong.

 

[Bandersnatch]

@Jacob T Anderson CWattters is right. I missed what you did there. (it's just not how you write it out correctly)

 

[Jacob T Anderson]

I still don't understand what do I do next? I tried dividing both sides by 4.905 and get t = 8.15, then I square rooting 4.905 which gives me 24.05 then tried dividing both of them again but this time with 24.05 which gave me t = 1.66. I feel retarded

 

[jbriggs444]

I tried dividing both sides by 4.905 and get t = 8.15

Show your work.

 

[Jacob T Anderson]

I got 8.15 not 8.13 oops

 

[Jacob T Anderson]

Sorry about the picture

 

[Bandersnatch]

It's all good until the very last step, where you wrote t=8.13. What happened to the $t^2$ you had in the previous step?

 

[Jacob T Anderson]

I got it! I forgot about the Quadratic Formula, thanks guys for the help.

 

[jbriggs444]

I got it! I forgot about the Quadratic Formula

Simpler than that. Take the square root of both sides.

 

[CWatters]

+1 No need to use the quadratic equation.